Niobium base high temperature alloys



United States Patent 2,881,069 NIOBIUM BASE HIGH TEMPERATURE ALLOYS Thor N. Rhodin, Jr., Wilmington, Del., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Application November 14, 1956 Serial No. 621,996

10 Claims. (Cl. 75174) This invention relates to novel nobium-base alloys and more particularly to improved niobium-aluminummolybdenum alloys exhibiting unusual strength and oxidation resistance under extreme high-temperature service conditions.

' For an alloy to be useful as a material of construction in applications such as jet and diesel engines, atomic power reactors, gas turbines, turbine blades or buckets and nozzle guide vanes for turbines, dies for high-temperature metal working, high-temperature reactors, and the like, it must possess satisfactory high melting point, strength and oxidation-resistance properties and also must be amenable to fabrication. Disadvantageously, prior metals and alloys lack these essential qualities to render existent a real need for an alloy capable of satisfactory performance under the service conditions encountered in applications of the type mentioned.

It is among the objects of this invention to overcome these disadvantages of prior metallic construction materials and to provide a novel alloy composition which is particularly adapted and useful for attaining these objects. Further objects of the invention include the provision of an improved, workable alloy composition having superior strength and oxidation resistance characteristics at relatively high, above 1000" C. temperatures; the provision of niobium-base alloy useful in the applications mentioned and adapted to withstand high mechanical stresses at temperatures above 1000 C.; to provide a niobium-aluminum-molybdenum alloy which is desirably resistant to oxidation at temperatures considerably above 800 C., and in a range of 1000-1300 C. or higher, and which is satisfactorily ductile and readily amenable to mechanical fabrication under hot or cold working or drawing conditions, including hot swaging, hot rolling, forging, extrusion, hot pressing, etc.; to provide a niobium-aluminum-molybdenum alloy having requisite, superior hardness properties and which does not require thermal treatment to develop maximum strength at elevated temperatures; to provide an alloy composition of the type mentioned having superior fatigue, tensile and rupture properties at relatively high temperatures and which undergoes no significant permanent dimensional changes upon subjection to prolonged exposure under extreme temperature conditions; and to provide' a niobium-aluminum-molybdenum alloy composition having unique, especially protective surface layers of reaction products and which advantageously exhibits such chemical and mechanical attributes as desired adherence, noupermeability, dimensional stability, nonvolatility and minimum film thickness when exposed to corrosive atmospheres at high temperatures. Other objects andadvantages of the invention will be evident from the ensuing detailed description.

These and other objects are realized by the alloys of this invention which contain as essential ingredients at least 55% by weight of niobium,about 20% by weight of aluminum and above 5% and up to 20% by weight of molybdenum. In combination with these ingredients 2,881,069 Patented Apr. 7, .1959

and to impart to the alloy certain desired characteristics, such as the properties of protective oxide scale or the special metallurgical response of the alloy to working,

such as for example, heat treatment or fabrication, addition can be effected of from 0-20% by weight of one or more of the elements iron, cobalt, chromium, nickel,

tungsten and zirconium; from 05% by weight of one or.

In a more specific and preferred embodiment, mynovel alloy composition contains from about 55% to by Weight of niobium, from 5% to 20% by weight of aluminum, and from above 5% to 20% by Weight of molybdenum. The amounts of said added elements range in total amounts: from 0-20% by weight of at least one of said iron, nickel, chromium, tungsten, zirconium and cobalt ingredients; the total of this group not to exceed 35%; from 05% by weight of at least one of said' beryllium, manganese, silicon, thorium, vanadium element-s, the total of this group not to exceed 15%; and from 02% by weight of at least one of the elements boron, carbon, calcium and cerium, the total of this group not to exceed 5%.

My improved alloys can be prepared in accordance with conventional procedures and through recourse. to known melting and casting techniques. Thus, the individual metals can be melt-cast together and the melt allowed to cool and solidify into a desired shape. The melting operation can be carried out in an arc melting furnace provided with consumable or nonconsumable electrodes, or by subjecting the charge to induction heating in a skull or crucible type of container. One useful form of arc melting furnace comprises that having an integral, water-cooled copper crucible in which the charge can be melted and solidified, such as that described by W. Kroll in Transactions of the Electrochemical Society, vol. 78, pages 35-47, 1940. Alternatively, a compressed, consumable arc electrode type melting furnace can be employed, such as described in US. 2,640,860 to S. A. Herres, as can the combination of a nonconsumable and consumable electrode type of double melting furnacedescribed in US. 2,541,764 to S. A. Herres. A continuous-feed type of furnace can also be used, such as de' scribed in US. P.B. Report 111,083. Whatever the type of furnacing means employed, care should beexercised in the melting and casting operation to protect the molten metal from normal atmospheric contamination through contact with oxygen, nitrogen, etc. This can be prevented by conducting the operation under a vacuum or an atmosphere of an inert gas, such as argon, helium, etc.

The individual metals charged to the melting furnace can be in any desired form, e.g., powder, granular, shot, wire or sponge, and should be of commercially acceptable purity to insure production of a satisfactorily pure alloy product. The cast material obtained will consist of a workable metal having excellent strength and oxidation resistance at high temperature, and will be eminently suitable for use as a material of construction in high-temperature equipment designed to operate at temperatures beyond the limits of present equipment constructed of the these temperatures other high-temperature alloys lose I strength, become plastic or melt. Also, they. are charac-.

terized by especially protective layers of reaction products on or below their metal surface which consist of compounds of the alloy with oxygen, nitrogen, hydrogen, carbon, sulfur or halogens or compounds thereof present in the atmosphere. Advantageously, they will be found to have been adjusted to produce especially protective surface layers containing combinations of the compounds mentioned with themselves, such as spinel oxides or with each other, such as mixed oxides-nitrides to provide very high resistance to deleterious attack of the alloy by the surrounding gases. The data given below demonstrate their properties in respect to high-temperature oxidation resistance. Their performance with reference to a balance between oxidation resistance and fabricability is determined by the relative proportions of the alloying elements. Since these two properties tend to oppose each other, the ranges in composition given were chosen on the basis of establishing an optimum compromise between them.

To a clearer understanding of the invention, the following specific examples, in which the percentages mentioned are by weight, are given. These are only illustrative and are not in limitation of the scope and underlying principles of the invention. As will be noted, these aluminum-molybdenum-modified niobium base alloys are characterized by unusually high niobium content, and in the preferred 55-80% range. In addition, the protective films characteristic of oxidation at 1000 and 1200 C. often contain relatively little nibium oxide but are usually substantially enriched in aluminum oxide. This inversion in the niobium-aluminum ratio between the alloy and the protective films is especially unique and comprises a novel and outstanding feature of my alloys.

EXAMPLE I A granular mixture of 60% niobium, aluminum, and 20% molybdenum was charged into a water-cooled, copper crucible of an arc melting furnace of the type described above and the metals heated under an atmosphere of helium to efiect complete fusion of the metal charge. When the charge became liquefied, the furnace was turned off and the melt was allowed to cool in the inert atmosphere, discharged from the crucible and was tested for resistance to high-temperature oxidation in the following manner:

A coupon was cut from the as cast button and heated at 1000 C. and 1200 C. for 24 hours in a helium atmosphere. The sample was then heated to 1000 C. and I200 C. in a recording thermobalance in flowing air for 24 hours. Oxidation rates were followed by making continuous measurements of weight change while the sample was at a controlled temperature without interruption of the test during the 24-hour period. Nonvolatility of surface compounds under these test conditions was also determined by measuring no weight change when exposed to pure helium.

Upon termination of the oxidation test, the sample was cooled and the protective character of the surface layers determined by metallographic examination and chemical analysis. In addition, the effect of oxidation on the metal alloy itself was examined by the same methods. It had an oxidation rate of 0.08 mg./sq. cm./hr. after 24 hours at 1000 C. and a rate of 0.62 mg./sq. cm./hr. after 24 hours at 1200" C. A specimen of pure niobium metal, subjected to the same test had, in contrast, an oxidation rate of 22.0 rug/sq. cm./hr. after 24 hours at 1000 C., and 68 mg./sq. cm./hr. at 1200 C.; and in some cases, was completely converted to the oxide after treatment at 1000" C. and 1200 C. The specimen of this example, on the other hand, was covered with a very thin, especially adherent protective oxide layer which corresponded to less than 0.10% conversion of the metal at 1000 C. and less than 0.31% conversion at 1200 C. This layer showed outstanding resistance to spallin'g when the specimen was heated at 1000 C. and 1200 C. and then cooled to room temperature. Upon forging and machining the remaining casting into a nozzle element and employing such nozzle for spraying MgCl at a temperature above 800 C. in a chemical process, the alloy exhibited excellent high-temperature oxidation resistance characteristics and proved efiectively useful in such application.

EXAMPLE II An alloy was prepared in the same manner as in Example I except that its composition was 68% niobium, 15% aluminum, 15% molybdenum and 2% cerium.

After testing as described in Example I, its oxidation rate was 0.05 mg./sq. cm./hr. after 24 hours at 1000' C. and 0.45 mg./ sq. cm./ hr. after 24 hours at 1200 C. This corresponded to less than 0.05% and 0.19% conversion of the metal at 1000 C. and 1200 C. respectively. The adherence and coherency of the oxide film in this alloy was found to be exceptional particularly upon heating and cooling from 1200 C. or from 1000 C. to room temperature.

EXAMPLE III An alloy was prepared as described in Example I except that its composition was niobium, 10% aluminum, and 10% molybdenum. A coupon was cut from the cast material and upon heating and testing as in Example I had the properties shown in Table 1 below.

EXAMPLE IV An alloy was prepared as described in Example I except that its composition comprised 60% niobium, 15% aluminum, 20% molybdenum and 5% tungsten. Upon testing as described in Example I, it had the properties shown in Table I below.

EXAMPLE VI An alloy was prepared as described in Example I except that its composition comprised 56% niobium, 14% aluminum, 15 of molybdenum, 10% zirconium, 4% tungsten, and 1% of boron. Its characteristics are shown in Table I below.

Table I Oxidation rate (mg/sq. cm./hr.) Adherence of Example oxide scale Control 2 Nb I. 60% Nb, 20% Al, 20%

2 68 Very poor. Mo 0. 08 0. 62 Good. II. 68% Nb, 15%Al, 15% Mo, 2% Ce. 0. 05 0. 45 Excellent III. 80% Nb, 10% Al,10% Mo 0.82 Fair. IVC. 65% Nb, 10% Al, 10% Mo, 15% 0. 24 0.95 Good 1. V. 60% Nb, 15% Al, 20% Mo, 5% W- 0. 40 Do. VI. 56% Nb, 14% Al, 15% Mo, 10% 0. 12 0. 32 Excellent.

Zr, 4% W, 1% B.

As noted, the alloys of this invention are useful as materials of construction in all applications requiring strength and a corrosion-resistant metal. Hence, while particularly useful in high-temperature equipment which must operate above 800 C., such as jet engine parts, nuclear reactors, gas turbine parts, etc., my novel alloys due to their outstanding properties, including nonbrittlencss and adaptability for successful fabrication by hot swaging or rolling, forging, hot pressing, or extrusion, are not restricted to such applications nor to any particular equipment described or referred to herein.

When using the elements iron, cobalt, nickel, chromium, tungsten and zirconium in combination with the contemplated amounts of niobium, aluminum and molybdenum, from about 1-20% by weight of such elements can be resorted to; when the elements beryllium, manganese, silicon, thorium, and vanadium are used, amounts from 0.1-5% by weight can be used; while 0.1-2% by weight amounts of boron, carbon, calcium and cerium can be utilized. In the instance of molybdenum employment, I contemplate a specific range amount of from about 6-20%.

Although preferably metals exhibiting relatively high purity are utilized herein, some variance in purity properties can be tolerated. Thus, the alloys of the examples and those tested were prepared from commercially available niobium, aluminum and molybdenum containing less than 1% incidental impurities. Commercial niobium usually contains tantalum (in amounts up to 5%) which is difiicult to detect and separate. Therefore, the niobium used herein may contain small amounts (0.1 to 5.0%) of tantalum, as well as iron, oxygen and possibly silicon as impurities. Elimination of certain of these impurities such as oxygen, or the enhancement of others such as tantalum or iron, may improve oxidation resistance significautly.

Since many changes and modifications can be made in the invention without departing from its underlying principles, it will be understood that it is not restricted to the above detailed description, but only as defined in the appended claims.

I claim:

1. A niobium-base alloy consisting essentially of at least 55% by weight of niobium, from 520% by weight of aluminum and in excess of 5% and up to 20% by weight of molybdenum.

2. An oxidation-resistant niobium base alloy composition consisting essentially of from about 55-80% by weight of niobium, from about 5-20% by weight of aluminum and from above 5 to 20% by weight of molybdenum, said alloy being adapted to withstand prolonged exposure at a temperature above 800 C.

3. A niobium-base alloy consisting essentially of at least 55% by weight of niobium, from 520% by weight of aluminum, from above 5 to 20% by weight of molybdenum, and in combination therewith from 0-20% by weight of an element selected from the group consisting of iron, cobalt, chromium, nickel, tungsten, zirconium, from 0-5% by weight of an element selected from the group consisting of beryllium, manganese, silicon, thorium, and vanadium, and from 0-2% by weight of an element selected from the group consisting of boron, carbon, calcium and cerium.

4. An oxidation-resistant niobium-base alloy composition consisting essentially of from about 80% by weight of niobium, from about 520% by weight of aluminum, from above 5 to 20% by weight of molybdenum, and from 1-20% by weight of chromium, said alloy being adapted to withstand prolonged exposure at a temperature above 800 C.

5. An oxidation-resistant niobium-base alloy composition consisting essentially of from about 55-80% by weight of niobium, from about 5 to 20% by weight of aluminum, above 5 to 20% by weight of molybdenum, and from l-20% by weight of zirconium, said alloy being adapted to withstand prolonged exposure at a temperature above 800 C.

6. An oxidation-resistant niobium-base alloy composition consisting essentially of from about 55-80% by weight of niobium, from about 520% by weight of aluminum, from above 5 to 20% by weight of molybdenum, from l20% by weight of zirconium, from 0.1-5 by weight of tungsten, and from 0.l-2% by weight of cerium, said alloy being adapted to withstand prolonged exposure at a temperature above 800 C.

7. An oxidation-resistant niobium-based alloy containing by weight, about niobium, about 20% aluminum and 20% of molybdenum.

8. An oxidation-resistant niobium-base alloy containing by weight about 68% niobium, about 15% aluminum, about 15 molybdenum and 2% cerium.

9. An oxidation-resistant niobium-base alloy containing by weight about 56% niobium, about 14% aluminum, about 15% molybdenum, about 10% zirconium, about 4% tungsten and about 1% boron.

10. A niobium-base alloy consisting essentially of at least 55% by weight of niobium, from 5-20% by weight of aluminum, from above 5 to 20% by weight of molybdenum, and in combination therewith from 020% by weight of at least one element selected from the group consisting of iron, cobalt, chromium, nickel, tungsten and zirconium, the total of this group not to exceed 35%: from 0-5% by weight of at least one element selected from the group consisting of berllium, manganese, silicon, thorium, and vanadium, the total of this group not to exceed 15%; and from 02% by weight of at least one element selected from the group consisting boron, carbon, calcium and cerium, the total of this group not to exceed 5% References Cited in the file of this patent Initial Investigation of Niobium and Niobium-Base Alloys, Saller, Stacy, Porembka, AEC, BMI-l003, May 23, 1955.

UNITED STATES PATENT OFFICE vI CERTIFICATE OF CORRECTION I Patent N0o 2,881,069 April '7, 1959 Thor Na Rhodin, Jr.

It is hereby certified that error appears in the-printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 6, line 10, before "above" insert from line 22, for "based" read base Signed and sealed this 5th day of April 1960.

(SEAL) Attest:

KARL Ho AXLINE ROBERT c. wATsoN Attesting Officer Commissioner of Patents 

10. A NIOBIUM-BASE ALLOY CONSISTING ESSENTIALLY OF AT LEAST 55% BY WEIGHT OF NIOBIUM, FROM 5-20% BY WEIGHT OF MOLYBOF ALUMINUM, FROM ABOVE 5 TO 20% BY WEIGHT OF MOLYBDENUM, AND IN COMBINATION THEREWITH FROM 0-20% BY WEIGHT OF AT LEAST ONE ELEMENT SELECTED FROM THE GROUP CONSISTING OF IRON, COBALT, CHROMIUM, NICKEL, TUNGSTEN AND ZIRCONIUM, THE TOTAL OF THIS GROUP NOT TO EXCEED 35% FROM 0-5% BY WEIGHT OF AT LEAST ONE ELEMENT SELECTED FROM THE GROUP CONSISTING OF BERYLLIUM, MANGENESE, SILICON, THORIUM, AND VANADIUM. THE TOTAL OF THIS GROUP NOT TO EXCEED 15% AND FROM 0-2% BY WEIGHT OF AT LEAST ONE ELEMENT SELECTED FROM THE GROUP CONSISTING BORON CARBON, CALCIUM, AND CERIUM, THE TOTAL OF THIS GROUP NOT TO EXCEED 5%. 